S. IcMkawa — So77ie Notes on Japanese Minerals. 63 



Art. YII. — So7ne Notes 07i Japanese Minerals ; by Shim- 



MATSU ICHIKAWA.* 



Y. Natural Etching of Garnet Crystals. 



Althoug^h crystals of garnet are found abundantly at many 

 localities, the natural etchings of the crystals have not yet been 

 discussed by mineralogists. In 1908, 1 collected garnet crystals 

 from Wadatoge, Sinano Province, and at this time observed 

 crystals with natural etchings. The results of the study of these 

 specimens are shown in the accompanying figures (I). 



Garnet crystals from Wadatoge occur in cavities of a vitreous 

 andesite. Their color is black, luster vitreous, and crystal faces 

 show the combination of the dodecahedron {d) and icositetra- 

 hedron {n) ; the habit varies according to which of these forms 

 predominates (see figs. 1 and 2). Crystals are mostly imperfect 

 and model crystals very rare : the etched crystals were collected 

 from earth produced by the decomposition of a white, tufaceous 

 rock accompanying obsidian ; they measured 3 to O"""" in 

 diameter ; etched figures were more frequent in imperfect than 

 perfect crystals. Some of the crystals were so much etched as 

 to be round like balls ; in general etched crystals are distin- 

 guished by their stronger luster. The rounded edges of the 

 etched crystals can be barely observed by the naked eye, and 

 the pits, elevations, etc., can only be investigated minutely 

 under a magnification of 75 to 140 diameters. 



Fig. 1 shows the natural etching of an icositetrahedral crys- 

 tal ; details are given in figs. 3 and 4. Fig. 2 shows a crystal 

 of dodecahedral type. Fig. 3 is a horizontal projection on the 

 principal axis of fig. 1 ; the solid angles at the extremities of 

 the axes are rounded by etching and elevations having the form 

 of an octagonal pyramid are found on their surfaces. The 

 cruciform edges on the extremities of the same axes are rounded 

 or hollowed and similar elevations formed on the ridges or 

 grooves. The edges formed by the combination of d and n 

 are transformed by faces of the hexoctahedron, mOn, by solu- 

 tion. 



Fig. 4 is a horizontal projection on the trigonal inter-axis of 

 fig. 1 ; the triplane solid-aiagles on the extremities of the axes 

 are leveled as faces of the octahedron by etching and pits of 

 the form of a hexagonal pyramid formed on their surface ; the 

 pits appear as negative-crystals on the hexaplane solid angles 

 on the trigonal inter-axes of the hexoctahedron. The trif urcate 

 edges on the extremities of the same axes are cut vertically, as 

 — \R^ by etching, and the new faces show neither pits nor ele- 

 vations, but sometimes striations are noted. 



*For an earlier paper, see vol. xlii, pp. 111-119, August, 1916. 



